Ricoh FW830, Service Manual

Page: 63 / 227

10.3.2 Overheat Protection
The overheat protection circuit on the main board turns off the fusing lamp if
the temperature goes too high.
The input from TH1 goes to pin 5 of IC107 as well as to the CPU. As the
temperature of TH1 increases, its resistance decreases and therefore the
voltage at pin 5 of IC107 increases. If this voltage becomes greater than the
voltage at pin 6, the output at pin 7 becomes HIGH. Q101 then turns off and
cuts the +24 volt supply to ac controller 2. The fusing lamp then turns off. The
overheat protection circuit becomes effective at about 220 degrees.
TF1 (2 thermofuses in parallel) provides overheat protection in case a short
bypasses the control and drive circuits. These two fuses will open if they
reach approximately 192 degrees.
FUSING AND PAPER EXIT 31 December 1992
2-44